As mentioned in the other thread, I have started a new 3 way project. The Vifa NE225W-4 woofers will be in separate bass bins. The PLA200-6 planar drivers and RT1.3WE tweeter will sit on top. The top enclosure will employ 2 special features. Firstly they will have 2.5" radius round overs on their vertical sides. I obtained some 5" diameter card board tubes that I cut in half and will glue to the sides of the enclosures (like I did in my "Trials" project). Secondly the enclosure panels will be very well damped. I will adhere pieces of ceramic floor tile to internal panels where appropriate. I will also fill the cavities formed by the card board tube halves with dried play sand or kitty litter.
I use a product called Sika Pro Select to adhere the tile pieces. It's actually called a construction sealant. It is water based so no smell and clean up is soap/water. When it's cured it is still very pliable. Here are some pictures of the rear panel. I didn't cover the entire panel because I needed a spot at the bottom for the Speakon connector. I applied the Sika to both the MDF and to the backside of the ceramic tile. There is about 1/8 inch layer of Sika.